An antenna device that transmits/receives an electromagnetic wave by utilizing a slot formed in a conductor layer is known. In such an antenna device, a radio-frequency current induced around the slot leaks out to an end part of the conductor layer, whereby an undesired diffraction wave is generated from the end part of the conductor layer. This diffraction wave deteriorates the antenna characteristics. In the antenna device, it is desired that the antenna characteristics can be prevented from being deteriorated.